1. Field of the Invention
This invention relates to digital data communications and, more particularly, to a communications system and method for providing spread spectrum data transfer at user-selectable transfer rates. In one embodiment, the invention uses direct sequence spread spectrum (DSSS) signaling.
2. Description of the Related Art
Anticipated data communications needs in industry, government, and educational institutions will require communications systems providing fast, high quality data transfer with substantial user flexibility. Because all communications media (e.g., electromagnetic spectrum allocations) are finite resources, a growing need exists for new technologies that use those resources more efficiently. Promising approaches for more efficient data transmission generally use some method of sharing the medium among multiple users, or multiple access signaling.
Some features desirable for multiple access systems have been realized through the use of time division multiple access (TDMA) signaling methods. TDMA systems allocate transmission capacity among multiple users within a transmission region by dividing transmission time into a series of discrete data frames and assigning to each user a specified time slot in each frame.
Through TDMA, multiple user stations can access a single passband carrier because all stations are precisely synchronized and each transmits only during its preassigned time slots. Many digital cellular telephone systems use TDMA signaling. A multiple access system also typically supports several discrete frequency carriers, each of which occupies a narrow frequency passband of the system""s spectrum allocation.
Although TDMA provides effective multiple access, it has several drawbacks relative to satisfying emerging data transmission needs. TDMA signaling with high transmission quality (i.e., low error rates) requires user receivers to perform frequent frame/slot synchronization. This, in turn, reduces signaling capacity and increases the complexity of user and base station equipment.
Timewise allocation of transmission carrier access also limits TDMA user transmitters to low duty cycles, which creates various negative consequences for battery life in mobile units and for transmitter hardware generally. Unintended recipients can easily intercept unencrypted TDMA transmissions. Moreover, time division complicates allocation of unused carrier capacity to users currently accessing the system.
An alternative to TDMA for multiple user access may be provided by spread spectrum techniques such as code division multiple access (CDMA) signaling. Spread spectrum techniques include modulation schemes that generate a transmission signal covering more bandwidth than is required for the message being transmitted. The increased bandwidth depends upon a message-independent spreading technique. Typical spreading techniques modulate the message signal over the transmission band in an unpredictable manner, thereby preventing intelligible reception of the message by other than its intended recipient.
CDMA signaling spreads a baseband data stream by modulating the data stream across the transmission passband (or xe2x80x9cfrequency carrierxe2x80x9d) in accordance with a pseudorandom code. A typical CDMA implementation involves direct sequence spread spectrum (DSSS) modulation, in which the message signal is phase modulated directly by a periodic but pseudorandom binary sequence.
DSSS can provide multiple users with simultaneous access to the same frequency carrier if each user modulates with a different spreading code and no two of the codes have significant statistical cross-correlation. Independent message transmissions, spread by such approximately uncorrelated codes, can occupy the same spread-spectrum frequency carrier with insignificant interference between messages. Each transmission occupies the entire carrier, just as in TDMA signaling. Unlike TDMA, however, multiple users of a DSSS system do not pass the carrier access around in accordance with assigned time slots; instead, they all transmit simultaneously.
A DSSS receiver recovers a message signal from the (spread-spectrum) transmission signal by modulating the spread-spectrum signal with the same code used to spread the message signal. The low cross-correlations of the spreading codes ensures that spread-spectrum components associated with other messages will remain spread. This second application of the spreading code will extract (or xe2x80x9cde-spreadxe2x80x9d) the message signal originally modulated with that spreading code.
CDMA offers several significant benefits for data communications generally, including a 100% transmission duty cycle, simple data synchronization, straightforward reprogrammability, and some message privacy. (See Table 1.) DSSS in particular has received considerable interest from some digital cellular service providers for its high spectral efficiency. It has been suggested that this efficiency could potentially increase user capacity by 20-40 times over analog cellular systems.
For cellular applications, CDMA signaling also has drawbacks. Practical cellular service must provide reliable access to subscribed users throughout a designated service area. A cellular service provider does this by dividing the service area into cells and locating a base station at the center of each cell.
User terminals, such as mobile transceivers, typically have only limited transmission ranges due to limited signal strength. The strength of a user terminal""s signal received at a base station depends upon the distance between the user terminal and the base station. Cell size thus must be limited so that each mobile unit within the service area always has sufficient signal strength to communicate with at least one base station.
CDMA system performance is also strongly affected by signal attenuation within a given cell. For the present discussion, the phrase xe2x80x9cuser capacityxe2x80x9d of a frequency carrier in a multiple access system means the number of users the system can accommodate, concurrently, on the frequency carrier. User capacity typically depends primarily upon the width of the carrier passband. For CDMA systems, user capacity also varies inversely with the data rate, the bit error rate of the recovered signal (expressed in terms of the signal to noise ratio, or SNR), and the power of the received signal.
Assured access and acceptable bit error rate requirements demand that a CDMA cellular system trade off data rate for signal power. That is, a user relatively distant, or remote, from the nearest base station must communicate at a lower data rate than another user located near the base station. This trade off works in practice, but the low data rates required for remote locations (e.g., adjacent cell boundaries) can noticeably degrade voice quality.
Emerging data transfer applications increasingly require more flexible and efficient communications systems that provide high data capacity and on-demand access. Current CDMA communications architectures do not offer these features. Present systems prevent active users from accessing the systems""unused data transfer capacity; instead, they allocate high data rates to some users whose needs could be met with lower rates, while limiting other users, who need substantial data transfer capacity, to undesirably low data rates.
In current CDMA communications systems, there is an unmet need for a practical, multiple access architecture that realizes the potential spectral efficiency offered by CDMA signaling. Such architecture should provide multiple users with flexible access to unused data transfer capacity and should enable them to select data transfer rates in accordance with their needs.
The invention constitutes a spread spectrum communications apparatus comprising an m-sequence generation unit, a symbol generation unit, and a code modulation unit. The m-sequence generation unit receives a phase assignment signal, indicating an assigned phase of a predetermined m-sequence, and generates a sequence signal representing the assigned phase of the m-sequence.
The symbol generation unit generates, in accordance with the sequence signal, a coding signal representing a basic symbol of an assigned signaling alphabet. The assigned signaling alphabet is selected from a predetermined number of signaling alphabets comprising respective basic symbols constructed from the assigned phase. The predetermined number of signaling alphabets have low cross-correlation therebetween and provide spread spectrum data transfer at an assigned transfer rate selected from a plurality of offered transfer rates. The code modulation unit applies the coding signal to an incoming first signal to generate an outgoing second signal.
The invention also constitutes a spread spectrum communications method comprising the steps of generating a sequence signal, generating a coding signal in accordance with the sequence signal, and generating an outgoing signal. The sequence signal represents an assigned phase of a predetermined m-sequence. The coding signal represents a basic symbol of an assigned signaling alphabet, the assigned signaling alphabet being selected from a predetermined number of signaling alphabets comprising respective basic symbols constructed from the assigned phase.
The predetermined number of signaling alphabets have low cross-correlation therebetween and provide spread spectrum data transfer at an assigned transfer rate selected from a plurality of offered transfer rates. The outgoing signal is generated in accordance with the coding signal and an incoming signal.